1. Field of the Invention
This invention relates broadly to continuous metal casting. More particularly, this invention relates to a method and apparatus for controlling caster mold heat removal to prevent breakouts by varying casting speed. The invention may be used in all sizes of strand casting machines in which solidification of a shell with liquid core starts in single or multiple water-cooled mold faces.
2. Description of the Prior Art
Generally, it is desirable to operate a continuous caster of metal billets, slabs and the like at the highest speeds possible to meet maximum production and utilization goals. However, in practice only optimum strand throughput at a predetermined allowable speed prevails, rather than at maximum speed, so as to avoid major disruptions in continuous caster operations. One such major disruption occurs when an improperly solidified strand breaks out because of insufficient heat transfer in the mold.
It has been discovered that numerous factors exist which lead to insufficient heat transfer when casting molten steel for example. First, low mold metal level due to either loss of metal level control or a build-up of an excessive slag layer when using a mold powder. The low metal level causes a reduction of residence time in the mold and thus reducing heat transfer. Second, A build-up of A1.sub.2 0.sub.3 content of the mold slag during casting of aluminum-killed steel which causes mold slag to become viscous and gummy and results in a significant loss in mold heat transfer. Third, in a rectangular mold, loss of narrow face taper increasing a strand-mold gap which reduces heat transfer on one or both narrow faces. Fourth, too high a casting speed which reduces residence time of a thin walled shell in the mold, thereby causing insufficient amounts of heat removal. Fifth, excessive temperature of liquid metal entering the mold which increases the mold heat load. Sixth, casting of wider and thicker slabs which reduce the surface area per volume of metal cast, thus causing more mold surface area to be used to extract the metal superheat and reducing the area available for formation of solid skin.
Caster operators are taught that a thicker skin may be formed in a caster mold to prevent breakout by manually reducing casting speed. This is based on known mold heat removal rate QA per square inch of strand surface which is speed dependent. However, there is lacking a prior art relationship defining when mold heat removal rate is insufficient, or in what manner caster speed corrective action should be taken. J. Shipman et al in U.S. Pat. No. 4,006,633 disclose how to satisfactorily measure and determine mold heat removal rate in single and multiple mold coolant flow circuits, but lacks direction as to determining minimum level of mold heat removal rate as well as caster speed correction. Earlier prior art is similarly deficient, or if mold heat removal is suggested, it requires a comprehensive analysis and determination of strand thermal and stress profiles all along the strand beyond the mold which must be combined with mold parameters.